US TV network ABC also ordered a new spin-off show, Marvel's Agent Carter, based on Hayley Atwell's Captain America character.
Both series tie into the Marvel film series, which includes star-studded titles like Iron Man and Thor.
They add background to events on the big screen, providing an overarching story to the so-called Marvel Universe.
Agents Of Shield was created by Joss Whedon, who scripted the Avengers Assemble film, and previously created Buffy The Vampire Slayer and Firefly.
The drama resurrected the character of Agent Coulson, who was killed in Avengers Assemble, with his character's miraculous recovery the central mystery of the opening episodes.
Played by Clark Gregg, the character took charge of a team of law-enforcement agents, tasked with investigating super-human individuals and unusual events.
Eventually, the plot of the series overlapped with Captain America: The Winter Soldier, in which the SHIELD organisation was brought down from within.
When the series debuted in the US in September 2013, it garnered 12.12 million viewers, marking the biggest network drama debut in four years.
Audience figures fell by nearly 4 million between the first and second episode, while figures for recent episodes average 5.5 million.
In the UK, where it is shown on Channel 4, the show's audience has also plummeted - from 4.5 million in September 2013 to 1.1 million last week.
However, the series plays well with young men, a much sought-after demographic with advertisers, and its ratings tend to recover once time-delayed viewing is taken into account.
Agent Carter, which follows the story of secret agent Peggy Carter, is being hailed as a "bridge show", which will be screened during Agents Of Shields' mid-series hiatus.
Meanwhile, Marvel comics is preparing another four series - Daredevil, Jessica Jones, Iron Fist and Luke Cage - via its partnership with Netflix.
Despite struggling in the ratings, Marvel's Agents of Shield has been renewed for a second series.