He died at 0730 hours, party sources further said.
His body would be kept at the Delhi BJP headquarters, for party workers, to pay homage to him.
A veteran in Delhi politics, Mr Garg, registered his first political win in 2003 when he won from the Wazirpur Assembly constituency.
In 2008 Assembly elections, he lost to Hari Shanker Gupta of Congress but by only about three thousand votes.
With Mr Garg as Delhi BJP chief, party registered strong performances in local municipal body elections.
The party gave him an important responsibility in 2019 Lok Sabha elections to strengthen it in Delhi, the party sources added. (UNI)