Brown has signed a deal until the end of January after he began his third coaching spell at Tynecastle by working for months without pay.
The 62-year-old, who left his role as East Fife manager at the end of last season, went in to help manager Gary Locke on July 1, weeks after the club went into administration and were hit with a 15-point penalty.
Brown told the Scottish Premiership club's official website: "I didn't enjoy being out of the game and came here with the intentions of helping Gary and the club as much as I could.
"I have a lot of enthusiasm for the club and the job and I'm very excited about what's happening here at the moment.
"We have the whole season to keep chipping away at the teams above us. The young players have done exceptionally well so far and they will only get better - but it is not going to be an easy task."
Locke played under Brown and Jim Jefferies at all three of his professional clubs - Hearts, Bradford and Kilmarnock - and was delighted to see one of his mentors rewarded for his loyalty.
"I am over the moon," Locke said. "I have the utmost respect and admiration for Billy and to have him as a shoulder to lean on is a huge help.
"He knows everything there is to know about the Scottish game and to have someone of his enormous experience here is a boost."