Wing Biggs, who was recently named the club's supporters' player of the year after scoring eight tries to be Bath's top try scorer this term, has agreed a two-year deal that will keep him at The Rec until 2014.
Scrum-half McMillan, meanwhile, has signed a deal through to the end of next season after being frustrated by injuries during this campaign.
Biggs, 27, said: "It was an easy decision to extend my contract. I feel my game has really developed at Bath and I'm enjoying the style of rugby we're looking to play.
"There is a lot of ambition here and a lot more to come from us, and I'm really looking forward to being part of everything."
Bath are currently searching for a new head coach, with director of rugby Sir Ian McGeechan and forwards coach Martin Haag leaving at the end of the season.
Gary Gold and Michael Cheika are understood to be two of the leading candidates.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.