Before graduating from the Academy, he was commissioned to write the first Persian Opera, 

Rustic Festival, for the opening of the Tehran Opera house, Rudaki Hall.

Upon completion of his studies, Pejman returned to Iran and soon became one the most prolific and prominent composers of his time. Combining his Western Classical music mastery with traditional Iranian music and instruments, he became a significant force in the classical Iranian music scene. His compositions for the symphony orchestras, ballets, operas, chamber music, oratorios, and film music span over an impressive 60-year career. His works have been performed nationally and internationally, including in the United States, Canada, Austria, Czechoslovakia, South Africa, Belgium, and the United Kingdom.  Pejman is a winner of numerous national and international awards and was named the most important composer of the century by the House of Cinema in Iran. A commemorative stamp was issued in Pejman’s honor in 2017.  

As a young violinist with the Tehran Symphony Orchestra , he was awarded a scholarship to study composition at the Music Academy in Vienna . He studied composition with Thomas Christian David , Alfred Uhl , and Hans Jelineck , Friedrich Cerha  as a first-year student at the Academy, Pejman's compositions were performed by the Vienna Chamber Orchestra  and his orchestral work Rhapsody was performed by the Vienna Radio Symphony Orchestra . In his third year as a student, he was commissioned to write the first Persian Opera Rustic Festival for the opening of the Tehran Opera house , Rudaki Hall . Upon graduation from the Academy of Music in Vienna, Pejman returned to Iran in 1968, where he was commissioned to write the opera Hero of Sahand  ( Delāvar-e-Sahand ) which was based on legendary Persian hero Babak Khorramdin . The opera was premiered at Roudaki Hall  in 1968. Between 1969 and 1978, Pejman continued to compose symphonic works, operas, ballets, and also wrote many scores for motion pictures and television programs.

In 1976, three years before the Iranian revolution, Pejman moved to New York, where he entered Columbia University 's Doctor of Music program in New York and continued his studies with Buelant Arel , Vladimir Ushachevsky  and Jack Beeson .

In 1984, Pejman moved to Los Angeles and continued to compose and arrange music for jazz and pop ensembles, and motion pictures Threat, 1985 .

In 1992, Pejman was commissioned to write a cantata for choir and orchestra for the liberation of  Khorramshahr   Beh Yade Khorramshahr, 1992 . He was then commissioned to write music for a musical theater to be performed at  Vahdat Hall  (previously called Rudaki)  Mokhtar, 1993 . Since 1993 Pejman has been writing film music and composing for the orchestra and choir and released various soundtracks and CDs.