The 22-year-old will join Super League club Dalian Aerbin for an undisclosed fee, if he passes a medical examination within days.
"Dalian Aerbin came to us with a substantial fee for Daniel and he expressed his desire to head to China," Adelaide United's football director Michael Petrillo said in a statement on Monday.
"It was a decision we did not take lightly because Daniel is an integral part of our squad."
Mullen, expected to sign a two-year deal with the Chinese club, played 79 games for Adelaide, his hometown club.
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