The 30-year-old was named as skipper as Scotland head coach Andy Robinson revealed his 30-man squad for the tournament in New Zealand, which for Scotland begins with the Pool B opener against Romania in Invercargill on September 10.
Kellock, who has captained Scotland on eight occasions, said: "The last one's gone, this one it's a huge honour.
"It was a huge honour to make the squad and when Andy asked me to be captain it was definitely the peak of my career."
Kellock will be able to sympathise with his Glasgow team-mate Johnnie Beattie, the most experienced of the seven omitted players.
Robert Harley, David Denton, Ryan Grant, Greig Laidlaw, Ben Cairns and Jack Cuthbert also missed out on the final selection, but all seven will be placed on standby and called upon if required.
Beattie was a victim of the intense competition in the back row.
Kellock, too, will have a battle to make sure of his place in the starting line-up, with Richie Gray, Jim Hamilton and Nathan Hines also vying for the two second-row places.
Kellock added: "It's by far the hardest it's been since I've been in the Scotland squad to get that starting jersey."
British and Irish Lions Chris Cusiter and Euan Murray proved their fitness to make the final playing party, while Chris Paterson is set to become the first Scot to play in four World Cups following appearances in 1999, 2003 and 2007.
Scotland's schedule, on paper at least, suggests the standard of opposition increases, with the Georgia game taking place on September 14, Argentina on September 25 and England on October 1.
Like Robinson and Kellock, Paterson is focusing on the first fixture.
The Edinburgh full-back said: "Romania and Georgia are two hugely physical sides, sides you really should hope to beat, but you have to beat them - they'll take a lot of wearing down."
Paterson made his debut in the 1999 tournament and is Scotland's most-capped player and record points scorer, with 105 appearances and 786 points.
He added: "It was a huge goal of mine to make this World Cup. I'm delighted to do that, but the hard work starts again when we get back to training and fight for that starting place."
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