Kosmos 421 was the forty-second of seventy nine DS-P1-Yu satellites to be launched, and the thirty-eighth of seventy two to successfully reach orbit. It was operated in an orbit with a perigee of 266 kilometres (165 mi), an apogee of 445 kilometres (277 mi), 70.9 degrees of inclination, and an orbital period of 91.65 minutes. It remained in orbit until it decayed and reentered the atmosphere on 8 November 1971.
Payloads are separated by bullets ( · ), launches by pipes ( | ). Manned flights are indicated in bold text. Uncatalogued launch failures are listed in italics. Payloads deployed from other spacecraft are denoted in brackets.