Mebrahtu was released by Melbourne Heart on Sunday and, as expected, the forward has signed a two-year deal at Parramatta Stadium.
Arguably more impressive is the capture of Mullen following his departure from Dalian Aerbin. The 24-year-old performed strongly during three months on loan at Melbourne Victory last season.
The arrival of a centre-back casts doubt on the futures of club captain Michael Beauchamp, out of contract at the end of the campaign, and Matthew Spiranovic, who has excelled for the Wanderers since joining on a one-year deal in October, but has already been linked with a move away from the club at the end of 2013-14.
Western Sydney coach Tony Popovic will face a significant challenge assembling a squad capable of matching the Wanderers' achievements during the first two years of their existence.
Marquee player Shinji Ono will return to Japan at the end of this season, while fellow key players Aaron Mooy and Youssouf Hersi are both out of contract and believed to be considering moves elsewhere.
The Wanderers' domestic squad is at capacity, meaning Mebrahtu and Mullen will not be eligible to play in the second half of the A-League season, but they have been registered for Western Sydney's AFC Champions League campaign.
