During their Chicago years, Jimmy and Marian also visited France in 1949 for the Paris Jazz Festival. This was semi-important for their association with the European jazz scene, but more significant because it marked the beginning of Marian's writing career. Marian's testimonial about the festival ran in the July 1949 issue of Down Beat.
In 1949, the McPartlands settled in Manhattan, living in an apartment in the same building as the Nordstrom Sisters. In 1950, she announced that she would no longer go by her stage name, Marian Page, but would now go by her married name, Marian McPartland. With Jimmy's help and encouragement, Marian started her own trio, which started performing at the newly opened 54th street club called The Embers on 8 May 1951. Here, she learned how to lead her own group, and played with greats such as Roy Eldridge, Coleman Hawkins, and Terry Gibbs. After trying out different combos, she settled on a trio of piano, bass, and drums that would soon become standard. This gig led to the notorious Leonard Feather review that opened by saying: "Oh, she'll never make it: she's English, white and a woman." She signed her first record deal without Jimmy in 1951, with Savoy Records. On 2 February 1952, McPartland opened a gig at the Hickory House that would continue regularly through November 1962. During her time at the Hickory House, Duke Ellington would often come in to listen. Ellington was influential on McPartland’s development as a pianist, and told her she played too many notes, a sentiment she would take to heart. The drummer Joe Morello joined the group in 1953 and was a member of the trio until he departed to join Dave Brubeck's Quartet in late 1956. In December 1953, Bill Crow replaced Vinnie Burke as her bass player. This trio of McPartland, Morello, and Crow would stay together through 1956, and be named Small Group of the Year by Metronome in 1954. The success of this trio would lead to the signing of McPartland to Capitol Records for five albums. The group of McPartland, Crow, and Morello would become McPartland's best-known trio. It has been argued that McPartland never received the acclaim she deserved because she never stayed with any sidemen long enough to develop a unique sound, her 1953-56 group being the exception to this rule.
McPartland continued writing testimonial pieces for journals such as Down Beat after the favorable reception of her first piece in 1949. Toward the end of the 1950s, she began to write about the issue of being a woman in jazz. She questioned "Can't we women make our own contribution to jazz by playing like women, but still capturing the essential elements of jazz – good beat – good ideas – honesty and true feeling?"