Until March 23, each game is available for $29.99 / £24.99, representing a 50% saving on their standard price once the launch discounts expire.
Its release marked the first time that a new series entry had arrived on Steam since Call of Duty: WW2 came out in November 2017.
It was soon followed by the release of Call of Duty Warzone 2 last November.
Sony, which is the main opponent of the deal, has suggested Microsoft could release degraded versions of Call of Duty games for PlayStation consoles if the Xbox maker acquires Activision.
Microsoft recently said it believes Call of Duty: Warzone and Modern Warfare 2 could be optimised to run natively on the Nintendo Switch.