Evans completed his service in February, 1946. He went to California to visit his mother and friends, and took a train cross-country to New York City, “ to meet his heroes and make some contacts.”  . He had a job waiting for him with Thornhill, who had reorganized his big band. But he was really interested in checking out what had been brewing musically in New York during the war years--- bebop. Evans recalled: “I got off the train, got in a taxi and went right to 52nd Street…. I met Bud Powell, Erroll Garner, Ben Webster and Lester Young all on that first night—Dizzy had his big band. This whole thing on 52nd Street was fantastic.” Gil moved into a basement apartment right nearby at 14 West 55th Street. “It had a piano and a bed and a record player and a tape player,” he said. “I rented the place for two years. I never knew who was going to be there when I got home and I didn’t care.”

Gil’s apartment quickly became a gathering place for young musicians. Gerry Mulligan ended up living there, and the “regulars” included Lee Konitz, Johnny Carisi, George Russell and John Lewis. Gillespie often came by, as did Charlie Parker, who mostly slept when he came over.  Soon, the young trumpet player Miles Davis, who was playing with Charlie Parker’s quintet, started spending time there, as well. Evans and Davis set up a trade that led to a lifelong friendship. Evans wanted Davis’s permission to arrange Davis’s composition “Donna Lee” for the Thornhill band; Davis wanted to look over Gil’s scores, which were considered  as “the first fresh big-band inventions to come along since the great Ellington concertos of 1939-42.” George Russell once described the atmosphere at Gil’s apartment: “ It was like an esoteric school and Gil was the school master. The key thing to come out of it was that we were all encouraged to reach for the impossible... One would be lucky to see things through Gil’s eyes.”

The endless musical discussions at Gil’s place led to the formation of the Miles Davis Nonet, and several custom-made arrangements for the group by Mulligan, Lewis, Davis, Carisi and Evans. In 1949, the Nonet had a short-lived gig at the Royal Roost, where the billboard, at Davis’s request, included the names of the arrangers. Though Evans only wrote two of these arrangements, his writing for the Thornhill orchestra was the blueprint for the Nonet’s sound, which included French horn, trombone, tuba, trumpet, alto and baritone sax and rhythm section; the style was “fast and light and no vibrato.” Capitol Records recorded the arrangements as singles, between 1949 and 1950. The recordings were reissued in 1954 and again in 1957 as the now-classic album The Birth of the Cool.   Gerry Mulligan said: “We were already talking about a dream band, the arranger’s ideal band; that was what we were looking for.” Years later, Evans said of this music:  “I did not create the sound, Claude did. I did more or less match the sound with the different movements by people like Lester, Charlie and Dizzy in which I was interested—it was their rhythmic and harmonic revolutions that interested me…The sound of the Thornhill band became common property pretty fast, but that sound could be altered and modified in many ways by the various juxtapositions of instruments. After those records [with the Nonet] what we had done seemed to appeal to other arrangers.” 

Evans married Lillian Grace in 1950. By then the Thornhill band was reduced to eight or nine pieces, and Evans’s work for the band had been winding down for about a year.  During the early 1950s, Evans often scuffled for work, but studied music of all description, and worked on his compositions and arrangements. He also worked with several singers, including Peggy Lee, and wrote two arrangements for Charlie Parker. In 1956, Evans’s career finally turned a corner, and he was getting more requests for arrangements from Billy Butterfield, Teddy Charles, and other New York musicians. He was asked to write two arrangements for Johnny Mathis’s first album, and vocalist Helen Merrill insisted that Evans be the arranger for her entire third album, Dream of You.  Even more significantly, he reconnected with Miles Davis, who had just signed on with Columbia Records. For one of Davis’s first albums,  producer George Avakian asked Davis and Evans to create an album that would expand upon some of the music of the Davis Nonet. Miles Ahead, recorded with the 19-piece Gil Evans Orchestra was their first large-scale collaboration.

The critical success of Miles Ahead cemented Davis’s and Evans collaborations over the next few years. In 1958, Evans and Davis recorded a stunning reinterpretation of George Gershwin’s opera Porgy and Bess, which became one of Davis’s best-selling records ever. Evans masterminded all the major roles for Davis, who was accompanied by an expanded jazz orchestra that included Cannonball Adderley, Paul Chambers, and the best studio musicians in New York City. Their next collaboration was Sketches of Spain, which was recorded over four sessions in late 1959 and March 1960. This album was another milestone for both musicians. The music, based on Rodrigo’s  Concierto de Aranjuez, and Evans’s reframing of flamenco sketches and the music of de Falla, defied categorization and caused a stir among critics. To Davis, Sketches was the most demanding of Evan’s scores: “to play parts on the trumpet where someone was supposed to be singing.”

At the same time, Evans kept developing his writing and work as a bandleader. In 1957, Evans  recorded his first album as a leader, Gil Evans Plus Ten, featuring soprano saxophonist Steve Lacy. The writing is tailored for Lacy as definitively as Evans’s writing for Davis. This ability—writing for particular individuals—enhanced Evans’s reputation, and brought forth several  new studio recording project including New Bottle, Old Wine, featuring Cannonball Adderley.
The rehearsal band Evans maintained in these years was also getting some visibility. The Gil Evans Orchestra  played at the Apollo Theater in 1959 on a bill with Dinah Washington and Thelonious Monks's Quartet. In 1960, the band  played a for six weeks at the Jazz Gallery, opposite John Coltrane’s quartet. Evans’s 1960 Impulse album Out of the Cool features the music that band performed nightly; much of this music is  a suspenseful balance of the written and improvised. From John Wilson’s review: “Evans is a full fledged member of that select group of composers-arrangers who have completely instinctive musical personalities...”