Thelma Grace Isabel Johannes began studying music at age eleven in the Leschetizky tradition. Early music teachers included Mrs. Robert Bell and George Palmer. Shortly after turning seventeen, Thelma achieved honors on her Performers' Associate examination. She gave her first public recital at the Normal School in Saskatoon, Saskatchewan.

Thelma then began training with pianist Dr. Lyell Gustin. Following subsequent recitals, Thelma and Horace Stovin, and later Hector Charlesworth, auditioned for work with the Canadian Broadcasting Corporation (CBC). Recitals frequently included Thelma's brother Earl, a violinist, and they performed in a variety of Saskatchewan communities including Watrous, Biggar, and Wakaw.

During 1943 and 1944, the duo of Edmund Assaly and Thelma Johannes met with considerable success as they played eighteen and twenty-four successive broadcasts with CBC. Through a scholarship, Thelma spent two years (1946-1948) in Paris, France studying music. She achieved degrees from the École Normale and the Paris Conservatory, studing with Robert Casadesus and Alfred Cortot. Thelma also spent two summers at the American Art School in Fontainebleau, France.

Upon her return to Canada, Thelma toured Saskatchewan and Ontario and performed on various CBC radio programs including Distinguished Artists, Masterworks for Piano, and Sunday Morning Recital. Thelma settled in Toronto for a year (1950) and worked as an accompanist for George Lambert, a leading vocal teacher at the Royal Conservatory of Music. Thelma then settled in Montreal, Quebec to resume her work with Edmund Assaly. For five years the duo played together on CBC and for the National Ballet of Canada. During this time Thelma also gave private lessons to students in the Montreal suburbs of Valois, Pointe Claire, and Bai D'Urfee. She modelled her teaching methods on those of Dr. Gustin.

In 1957, Thelma Johannes married James Victor O'Neill, formerly of Saskatoon, Saskatchewan. The two settled in Edmonton, Alberta where Thelma worked for the Alberta College Conservatory of Music. Thelma also taught private lessons and offered repertoire, technique, and pedagogy classes.

In addition to teaching, Thelma adjudicated music festivals in Alberta and Saskatchewan. Likewise, she received an invitation from the Western Board of Music to adjudicate the Saskatchewan Music Festival, and adjudicated the Saskatchewan Music Festival for three years. Thelma also accompanied the Edmonton Symphony Orchestra for several years, and shared Program Directorship of the Edmonton Chamber Music Society with Dr. Robert Strangeland for two years. The Canadian Federation of Music Teachers Association invited her to direct "The Young Artist Series", a program that she participated in as a youth. In 1981, Thelma completed her Bachelor of Music from the University of Alberta. Thelma O'Neill died on June 27, 2003.

Thelma O'Neill was with the Leschetizky Association since 1946. She also maintained membership with the Registered Music Teachers' Association in Edmonton since 1961, and held a variety of positions including Secretary, Vice-President, President (1966-1967), and National Council Representative.