Mahajan was playing his first match of the tournament but held his nerves to good effect in the last over when he was given the bowl with Heroes needing 15 runs to win the match.
He did not disappoint his captain and claimed two wickets off his first two deliveries which also accounted for Stuart Binny (50) who was the top scorer for Heroes.
T Sudhindra, who shared the new ball with Shane Bond, halted the Hyderabad Heroes innings immediately and reduced them to three for 15 in just the fourth over.
The pacer claimed three wickets for 23 runs in his alloted four overs and the target seemed a daunting one for the Heroes and ultimately, the Heroes failed to get their act together.
For Heroes, former South African all-rounder Justin Kemp chipped in with a useful 33 and was looking dangerous when Shane Bond removed him in his second spell.
Earlier, after winning the toss and electing to bat first, Delhi Giants were restricted to a modest 137 for the loss of eight wickets.
Abdur Razzaq produced a fiery spell of bowling as he claimed four wickets for just 23 runs and was well supported by former Kemp (1 for 17).
Kemp bowled a tight line to Giants' batsmen as they failed to negotiate him early in their innings.
Razzaq looked too strict with his line and length and his tally also included the wickets of Giants' captain Marvan Atapattu and hard hitting batsman JP Yadav.
Delhi Giants began their innings rather cautiously but opener Mohnis Mishra, in order to increase the tempo, lofted a Kemp delivery to fine leg, only to find Ambavati Rayudu waiting for the catch there.
Dhruv Mahajan joined Avishkar Gunawardene (28 runs) and added 61 runs in the next nine overs. Mahajan was the top scorer for Giants and made a gritty 36 off 33 balls which included four boundaries and a six.
Just when the Mahajan-Gunawardene partnership was looking threatening, left-arm spinner IS Reddy removed Mahajan with a well flighted delivery.
For Hyderabad Heroes, left-arm spinners Nickey Boje (23 in four overs) and IS Reddy (11 in two overs) produced crucial spells giving away to restrict the Giants for a modest 137. The Giants will now take on the Kolkata Tigers on March 29.
Delhi Giants: 137 for 8 in 20 overs (D Gunawardene st Khaleel b Boje 28, D Mahajan c Harris b Reddy 36, S Abbas Ali c Anirudh Singh b Absolem 16; Abdul Razzaq 4-0-23-4)
Hyderabad Heroes: 130 for 8 in 20 overs (Justin Kemp c Yagnik b Bond 33, Stuart Binny c Yadav b Mahajan 50, Chris Harris c A Sharma b Mahajan 20; TP Sudhindra 4-0-25-3, D Mahajan 2-0-13-2)