In July 1974, he joined the Indian Foreign Service (IFS) and worked as a Second Secretary in Jakarta. Later, he served as a First Secretary in Bangkok and Norway.
In June 1984, while posted in Norway, he resigned in protest against the Operation Blue Star.
As the Congress Government filed some cases against him, he decided to stay in Norway, running a small eating joint and working as a postman in Norway. He returned to India in 1990, after being assured that the cases against him would be dropped.
During 1990-91, he served as the Chairman of Punjab Human Rights Organization, an NGO that highlighted the excesses committed by the State Government during the anti-insurgency operations. During 1991-03, he served as a secretary of the Shiromani Gurdwara Prabhandak Committee (SGPC).
Harinder Singh joined Shiromani Akali Dal, and was elected as an MP from Bhatinda in 1996, winning by a margin of 92,229 votes.
He was a member of the third National Commission for SC/ST under the Atal Bihari Vajpayee Government. Aam Aadmi Party.
In 2014, Harinder Singh joined the Aam Aadmi Party, and contested the 2014 Lok Sabha elections from Fatehgarh Sahib.
He won by a margin of 54,144 votes. On 29 August 2015, he was suspended from the AAP due to anti-party activities.
He has denied any involvement in any such activities.