The 27-year-old lock, who missed the last international season through injury, had already extended his deal with the Queensland Reds and was therefore unlikely to join fellow Wallabies Drew Mitchell and Digby Ioane in heading to France.
Horwill will be one of the first names on the team sheet for the Lions clashes and is likely to lead his country into the highly-anticipated series.
"It's coming up quickly now and you can really feel that in the intensity it has brought to Super Rugby this year, especially in the derby games," he said.
"The competition just to make the squad is fierce, which is exactly as it should be.
"It bodes well for what is going to be the series of a lifetime, both for the test players but also those players who feature against the Lions in the tour matches.
"You just have to look at the power in the touring squad they have named to see that."
Horwill led Queensland to the 2011 Super Rugby title and then captained Australia for the first time when the Wallabies clinched the Tri-Nations title for the first time in a decade with a 25-20 win over the All Blacks.
Handed the captaincy on a permanent basis in preference to Rocky Elsom in a surprise move on the eve of the 2011 World Cup, Horwill led Australia to third place in New Zealand.
He had missed the entire 2010 international season with a knee problem and his injury woes returned last year with a freak incident in May leaving him with a serious hamstring tear.
Those absences mean he has played just 35 tests since his debut in 2007 but, if fit, he is sure to add to that tally in the three matches against the Lions in June and July.
"Obviously it's been frustrating at times, but you have to take the bad with the good," Horwill said. "The main thing is to focus on what's dead ahead - and that couldn't be more exciting.
"Playing the British and Irish Lions is a once in a lifetime opportunity and you can feel the excitement around the country, be it the players, the rugby community or the general public."