Mousa joined the Iraqi Communist Party in October 1959, and remained a member during the suppression of the party between 1965 and 1968. He was arrested in May 1968 due to his links with the party, although was released as part of an amnesty at the end of the year following the Ba'athist coup. He worked as an economic researcher in the national oil company from 1970 until late 1978 when he left Iraq. He had been appointed to the Central Committee of the party at the National Conference in 1976. He returned to Iraq in 1983, where he lived in the Kurdish-controlled north of the country and was involved in activities against Saddam Hussein's regime. In 1993, he became Secretary General of the Iraqi Communist Party.