Lee Gaskell grabbed the game's first try after 22 minutes, with Danny Brough adding the extras and a later penalty to make it 8-0 at half-time.
Ukuma Ta'ai's touchdown for the hosts was followed by Gaskell's second as Huddersfield built a 20-point lead.
Josh Drinkwater got Leigh's only try before Leroy Cudjoe completed the win.
Huddersfield's triumph, their second in as many weeks over the competition's bottom two sides, confirms the make up of the Super 8s and Qualifiers.
Warrington, who won the League Leaders' Shield last season and lost both major finals in 2016, are resigned to having to battle to avoid relegation.
Leigh, whose captain Micky Higham made his 500th career appearance in the defeat, need to beat near neighbours Salford next week and hope that Widnes fail to overcome Warrington if they are to avoid finishing bottom after 23 Super League games.
Huddersfield Giants boss Rick Stone:
"It was a tough, physical game and we showed some good resilience.
"Our goal is now to win as many games as we can and we have a few boys to come back from injury."
On confirming a top eight finish: "I was always confident as most of the time we were a bit banged up with injuries and I felt once we got those players back it would help us.
"It started probably at the Magic Weekend with a bit of a rusty win over Catalans. We began to respect the process a bit more - the boys have bought into that and the last couple of months have been good."
Leigh Centurions coach Neil Jukes:
"I thought we turned up for one another and had a real dig, but there were just too many silly scenarios which kept putting unforced pressure on ourselves.
"While the scoreline doesn't look great, to do the amount of defending we did shows that there's still a good work ethic within them. But regarding smartness we probably get one out of 10."
Huddersfield Giants: Rankin; McGillvary, Cudjoe, S Wood, McIntosh; Gaskell, Brough; Rapira, Leeming, Ikahihifo, Roberts, Ferguson, Hinchcliffe.
Replacements: O'Brien, Wakeman, Clough, Ta'ai.
Leigh Centurions: Brown; Higson, Crooks, Langi, Dawson; Mortimer, Drinkwater; Hock, Higham, Hansen, Paterson, Vea, Stewart.
Replacements: Pelissier, Tickle, Hopkins, Burr.
Referee: Scott Mikalauskas
Huddersfield Giants confirmed their Super League status for another season with a comfortable win over bottom side Leigh Centurions.