Dennis, 27, has won 15 caps for his country but did not feature for the Wallabies in their recent 2-1 test series loss to the British and Irish Lions.
"I really want to keep working hard to try and get back in the Test fold and contribute to winning a first ever Super Rugby title with the Waratahs," Dennis said in a statement on Sunday.
Inside centre Horne, 23, came off the bench in the second test victory over the Lions to win his 15th cap after enduring a difficult spell with injuries.
They join Wallaby forwards, Liam Gill, James Slipper and Rob Simmons in agreeing contract extensions this month.
Dennis, Gill, Slipper and Simmons were named in new Australia coach Ewen McKenzie's 40-man training squad for the upcoming Rugby Championship against New Zealand, South Africa and Argentina this week but Horne missed out on a place.
The Wallabies begin their campaign on Aug 17 at home to the All Blacks in Sydney.