The Australian international celebrated his 30th birthday with the winner in Shanghai SIPG's 2-1 victory over Wuhan Zall in the Chinese Super League.
"I've only been here a few days with the team so I didn't expect to play," said Mooy, who joined the Brazilians Oscar and Hulk at SIPG after moving from Brighton and Hove Albion.
"But I was ready in case the coach asked me to come on so I'm just happy I could come on and help the team."
Mooy, also formerly of Huddersfield Town and Manchester City -- where he failed to make an appearance -- came on as a substitute for the second half on Tuesday.
He said he was not 100 percent fit, having spent two weeks in coronavirus quarantine after landing in China, but the Australian was an immediate threat.
Former West Ham United striker Marko Arnautovic put SIPG ahead in the 59th minute, then ex-Chelsea star Oscar played in Mooy for a cool chip over the onrushing goalkeeper to make it 2-0 on 71 minutes.
Wuhan pulled a goal back but SIPG held firm. Vitor Pereira's side have lost only once in 11 matches this season, underlining their title credentials.
Mooy hopes his experience of playing international football in Asia, including in World Cup qualifiers, will help him settle in the CSL.
"With Australia we play in Asia and play against Asian teams so I sort of knew what the feeling was going to be like," he said, reflecting on his successful debut.
The CSL has been split into two groups as part of a rejig because of the coronavirus, with SIPG top of the eight teams playing in the city of Suzhou.