Glenn Zaleski grew up in Boylston, a small town in central Massachusetts, the younger son of Bob and Barbara Zaleski. He started piano lessons at seven and joined his elementary school's jazz band at ten. His older brother Mark, a clarinettist and saxophonist, recruited him into a working duo billed as the Zaleski Brothers, and the two played regularly around central Massachusetts while Glenn was still a schoolboy; by high school he was taking gigs in nearby Worcester. In 2004 he attended the Brubeck Institute's Summer Jazz Colony, and after graduating from Tahanto Regional High School in 2005 he won a two-year fellowship to the Brubeck Institute at the University of the Pacific in Stockton, California. He played alongside Dave Brubeck himself at the Monterey Jazz Festival in 2006. Two of the friends he made at the Institute, saxophonist Lucas Pino and drummer Cory Cox, had been his high school classmates, and the three of them moved to New York together.

Zaleski arrived in New York on his twentieth birthday and enrolled at The New School, finishing his degree in 2009. He stayed on for graduate work at NYU from 2009 to 2011, teaching classes and private students there at the same time. In 2011 he reached the semi-finals of the Thelonious Monk International Jazz Piano Competition and was a finalist for the APA Cole Porter Fellowship in Jazz. His first documented working band was a co-operative piano trio with bassist Rick Rosato and drummer Colin Stranahan, billed alphabetically as Stranahan/Zaleski/Rosato. The trio put out three albums on Capri: Anticipation (October 18, 2011), Limitless (recorded in New York on July 18, 2012 and issued October 15, 2013), and Live At Jazz Standard, taped over two nights at the Manhattan club on August 7 and 8, 2018 and released September 20, 2019. All three are collective dates with all three names on the sleeve rather than piano-trio records with a leader.

A reading session for new charts brought Zaleski into contact with Ravi Coltrane, who began calling him for gigs. Coltrane in turn introduced him to bassist Dezron Douglas, and when Zaleski was booked at the Side Door in Connecticut without his regular rhythm section he called Douglas and drummer Craig Weinrib, whom he had known since The New School. That one-off band became the group on his first record as sole leader, My Ideal, cut at Acoustic Studios in Brooklyn on June 10, 2014 and released by Sunnyside on March 17, 2015. It is a standards record built on the model of Bill Evans's Everybody Digs Bill Evans and Portrait in Jazz: seven canonical tunes taken without rehearsal, plus pieces by two friends, Rick Rosato and vibraphonist Peter Schlamb, and a closing "I'm Old Fashioned" arranged by Ravi Coltrane, who plays tenor on it. Fellowship, recorded May 3, 2016 with the same trio and released February 24, 2017, reversed the proportions: seven Zaleski originals against a Duke Pearson tune and John Coltrane's "Central Park West." Its titles are unusually literal about where he came from. "Table Talk" is named for the pie company near Boylston, "Westinghouse" is a tribute to Billy Strayhorn, and "Lifetime" is dedicated to a jazz club of that name in Shizuoka, Japan.

In 2018 Zaleski started his own label, Stark Terrace Music, and used it for two unaccompanied records that sit apart from the rest of his catalogue. Solo Vol. 1 (2018) collects standards and jazz-composer pieces including Charlie Parker's "Passport," Chick Corea's "Tones for Joan's Bones" and Brubeck's "Summer Song." Solo, Vol. 2, released May 20, 2022, came out of pandemic-era sessions at Acoustic Studios: three tracks from March 26, 2021 and three from February 15, 2022, drawn from about thirty-six songs he recorded across three unplanned two-hour sittings and released as videos to his Patreon subscribers. The programme runs to Wayne Shorter's "Ana Maria," Charlie Haden's "First Song," two Thelonious Monk pieces and Miles Davis's "Blue in Green," and Zaleski described its mood as more melancholy than the first volume.

The Question, recorded February 16, 2020 and released by Sunnyside on July 17, 2020, was his return to the quintet, the format he had favoured at The New School and set aside for a decade of trio and solo work. The band is Adam O'Farrill on trumpet, Lucas Pino on tenor, Desmond White on bass and Allan Mednard on drums, with two exceptions to the quintet instrumentation: "BK Bossa Nova" is a guitar feature for Yotam Silberstein, and the nine-piece "Subterfuge" adds Nick Finzer's trombone, Andrew Gutauskas's baritone, Alex LoRe's alto and Andrew Renfroe's guitar. Zaleski wrote most of the material during walks in the Vale of Cashmere in Brooklyn's Prospect Park in the summer of 2019, the season he turned thirty and learned he was about to become a father. The record opens with "The Question" and closes with "The Answer," the same melody resolved differently, and includes Brubeck's "Strange Meadow Lark" and a blues by James Williams.

A European tour with Cécile McLorin Salvant pulled him back onto the road after the pandemic and put her song "Monday" in front of him. He recorded the first instrumental version of it on Star Dreams, cut at Acoustic Studios on October 9, 2023 and released by Sunnyside on August 23, 2024. That album introduced his current trio with Dezron Douglas and Willie Jones III, a band he first assembled during the pandemic for a long run of dates at Mezzrow in Greenwich Village; he had come to know Jones through Ken Peplowski's group. Alongside "Monday" it carries Horace Silver's "Opus De Funk," Parker's "Passport," and a ballad called "Wayne," written in Paris shortly after Wayne Shorter's death in March 2023 and built on a pentatonic melody in Shorter's manner. The title track takes its name from a conversation with Zaleski's son about what he sees while asleep.

Across the same fifteen years Zaleski has become one of the most heavily booked pianists of his generation in New York, with more than fifty sideman credits. He is the pianist on both of Polish guitarist Rafal Sarnecki's New York albums and on all four of violinist Tomoko Omura's, on Nick Finzer's Outside In albums, on Ben Wendel and Phil Donkin's The Gate, on Chris Potter's date with violinist Scott Tixier, and on both of German drummer Mareike Wiening's Greenleaf albums. He has held the piano chair in Lucas Pino's No Net Nonet since its first recording in 2013 and in alto saxophonist Alex LoRe's band Weirdear across three albums. His bio names further associations with the Vanguard Jazz Orchestra, Melissa Aldana, Lage Lund and Ari Hoenig.

Zaleski names Bill Evans and Dave Brubeck as the two pianists he learned the repertoire from, and both marks are audible: Evans in the voicing and the unhurried rubato of his ballads, Brubeck in the willingness to build a whole performance out of a simple, singable head. Reviewing My Ideal in The New York Times in March 2015, Nate Chinen praised his touch and phrasing while warning that the album sat close to Brad Mehldau's territory and needed a stronger claim to originality, and the leader dates since have answered that in two directions at once: outward into his own composing, and inward into unaccompanied solo piano. He is unusually articulate about the writing side, and his stated aim on Fellowship was to leave room in the form for the band. As he put it: "Overwriting can stifle improvisation, but underwriting can result in monotony or chaos." What has made him a first-call sideman is a quality closer to discipline than to flash. NPR's Becca Pulliam wrote that he has a sound, JazzTimes credited his playing with a balance of excitement and coherence, and the bands that hire him repeatedly, from Lucas Pino's nonet to Ken Peplowski's swing-rooted quartet to Tomoko Omura's Japanese-folk-song project, span a range that few pianists of his generation cover.