His family moved to Western Australia when he was a child, initially living in the country before eventually settling in Perth.
Grewar attended Perth Boys School and Perth Modern School before going on to the University of Western Australia, where he studied agricultural science.
After graduating, he moved to Esperance, where he initially worked for the state Department of Agriculture.
Grewar later worked as a manager for a land development company and as a private consultant, as well as farming his own land.
He was elected to the Esperance Shire Council in 1971, and served until 1974.
Grewar entered parliament at the 1974 state election, defeating Bill Young (the sitting Country Party member) in the seat of Roe.
He was re-elected at the 1977 and 1980 elections, but his seat was abolished in a redistribution prior to the 1983 election.
Grewar attempted to transfer to the new seat of Esperance-Dundas, but was defeated by Julian Grill (the sitting Labor member for Yilgarn-Dundas, which had also been abolished).