...but, there was one man who lasted even longer, but, because Batman always had the cover, had never had a cover of Detective Comics while he was a back up feature in it.
The Martian Manhunter mostly hid his powers and alien nature for his earliest cases, he eventually did reveal his Martian form as a super-heroic identity in Detective Comics #273 (November, 1959), joining the Justice League of America soon after.
Zook. J'onn's Detective Comics run came to an end with Detective Comics #326 (April, 1964), moving over to House of Mystery, where he finally got some covers.
The Martian Manhunter finally did make a Detective Comics cover, #715 (November, 1997), the second part of when he teamed up with Batman to face Firefly.
The Sea King and his young ally, Aqualad, got a temporary home with Batman and the Martian Manhunter, running from Detective Comics #293 (July, 1961) to Detective Comics #300 (February, 1962), after which, he started his own comic for the first time!
Sadly, Aquaman never made the cover of Detective Comics, but the stories were drawn by Nick Cardy, who also did the earlier issues of Aquaman's first series, and so, a representative cover of the era was chosen.
The Elongated Man moved over from working with the Flash, and stayed in Detective Comics, getting a couple of costume changes, working with Green Lantern, the Atom, Zatanna, Zatara, Commissioner Gordon...
...as well as a couple of full issues team ups with Batman and Robin, as well as facing foes like Mirror Master, Chronos and the Riddler, until his continuous run ended with Detective Comics #383 (January, 1969).
The pair were traveling the country as Ralph used his Gingold powers to sniff out mysteries, including issues #426, #430, #436, #444, #449, #453, #456, #457, #462, #465, #488 and #500, as well as appearing with others in two issues that will come up!
Hawkman (and eventually his wife, Hawkgirl), would appear in as back-ups once in a while, including in issues #434, #446, #452, #454, #455, #467, #479, #480 and #500 (as well as in one issue with Elongated Man and more...which more will be said on soon).
Roy Raymond feature in Detective Comics #487 (December-January, 1979/1980, one of very few appearances of Hawkgirl without Hawkman) before the pair settled in World's Finest Comics, which eventually led to the Shadow War of the Hawkman, and then a regular comic series for Hawkman right around the time of the Crisis On Infinite Earths.
Hawkman and Hawkgirl didn't make many covers, so instead, here's a few from Brave and the Bold with Batman, including a foe introduced to Hawkman in Detective Comics, the Fadeaway Man.
The Atom then kicked off a run of back-ups, with the Calculator as the focus with Detective Comics #463 (September, 1976). Atom defeated him, but then appeared in the Black Canary back up of the next issue, and, along with all the heroes he faced in back ups (including Elongated Man, Green Arrow and Hawkman) in issues leading up to Detective Comics #468 (March-April, 1977)...
Calculator (Aquaman appeared briefly here as well, as did the head of Metropolis' WGBS, Morgan Edge), which can be seen on this Jim Aparo cover.
Atom also had one last back-up in Detective Comics, facing a foe from the JLA, the Dharlu, in Detective Comics #489 (April, 1980) before settling back in Action Comics (alternating with the second Air Wave and Aquaman, including another battle with the Calculator), then getting into the Sword of the Atom mini-series and specials around the time of the Crisis On Infinite Earths.
Green Arrow and Black Canary also faced Steelclaw in Star City (and a team-up with Batman and Catwoman), as their time in Detective Comics wore down, ending Detective Comics #567 (October, 1986), after which, she became part of the Justice League and Justice League International.
Green Arrow spent most of his time concentrating on being a columnist for the Daily Star.
Green Arrow got some odd villains at this time based on simple gimmicks, like Hi-Tek, the Executrix, Ozone, the Detonator, the Printer's Devil, the Death-Dealer, Bad Penny, the Pinball Wizard, and Vengeance...
....all before his back-ups ended with Detective Comics #567 (October, 1986), and Green Arrow would then get a new look and attitude with Green Arrow: The Longbow Hunters by Mike Grell.
...which also features back-up characters Human Target, which has already been covered, and Batgirl and Robin....who will be!