Ruth Watson Henderson was the recipient of many scholarships and awards while studying piano 1937–45 with Viggo Kihl and 1945–52 with Alberto Guerrero at the Toronto Conservatory of Music (Royal Conservatory of Music). She continued her studies 1952–4 with Hans Neumann in New York on scholarship from the Mannes College of Music. Her composition teachers included Oscar Morawetz, Richard Johnston, and Samuel Dolin. After her professional debut in Toronto in 1952 she was active as a concert pianist appearing frequently as soloist with Canadian orchestras and regularly on CBC radio. In 1956 she won the grand prize of the CBC's Opportunity Knocks. She taught and was an organist‑choirmaster 1957–61 in Winnipeg and 1962‐8 in Kitchener, Ontario, returning thereafter to Toronto.
Henderson began composing while accompanist of the Festival Singers 1968–79 and since then has been a prolific composer, writing more than eighty pieces for the choral repertory alone. Her harmonies often draw from modal or impressionistic influences and her rhythms closely follow the text she is setting. Many of her compositions continue to be popular in Canada and around the world, where she is often requested to accompany or adjudicate at festivals and competitions. Best known as a composer of choral music, Henderson has also written for organ, piano, string orchestra, and for winds, brass and percussion. Her Chromatic Partita for Organ was a prizewinner at the International Competition for Women Composers in Mannheim, Germany, in 1989.
She has received commissions from the Alliance for Canadian New Music Projects (Through the Eyes of Children), the Guelph Spring Festival (The Ballad of St George), the Oriana Singers (Toronto) (Songs of the Nativity), the Toronto Children's Chorus (The Last Straw, featuring tenor Ben Heppner), the Amadeus Choir of Scarborough (Voices of the Earth), and the Elora Festival (Five Ontario Folk Songs), among others. Henderson's more recent commissions include works for the twenty-fifth anniversary of the Contemporary Showcase in 1995; the 2002 national convention of the American Guild of Organists (Darkness to Light); the Mount Royal Kantorei (Magnificat); and Chorus Niagara's fortieth anniversary in 2003 (Voice of Niagara). Upcoming commissions include a work for the Bach Elgar Choir of Hamilton, combined with the Hamilton Children's Chorus.