Shaun Derry's men were completely outclassed by the Yorkshire club, who scored four goals in a wonderful first-half display.
Newcastle United loanees James Tavernier and Haris Vuckic put Rotherham two goals to the good, with the latter then scoring his second on the hour mark after an Alex Revell strike and a Kieran Agard penalty had put the game beyond doubt.
Tom Hitchcock added a sixth on 83 minutes as County succumbed to a humiliation that leaves them five points adrift of safety.
Fellow strugglers Stevenage enjoyed better fortune, forward Luke Freeman's second-half brace giving them a 3-2 win at Bradford City.
Striker James Hanson put Bradford 2-1 up just before the break after Francois Zoko had cancelled out Adam Reach's opener, only for Freeman to steal the show and move them off the bottom and above County.
Wolves and Leyton Orient remain neck and neck at the top of the division following home wins.
French forward Nouha Dicko scored a brace as league leaders Wolves beat Port Vale 3-0, while Moses Odubajo's late goal gave second-placed Orient a 2-1 triumph against Colchester.
Third-placed Brentford fell three points off the pace with a 0-0 draw at Carlisle United, but Preston North End continued their fine run of form with a 2-1 victory over Walsall.
An 83rd-minute winner from midfielder Simon Gillett boosted Bristol City's survival hopes with a 2-1 success over Gillingham, and Crawley Town pulled clear of danger as Matthew Tubbs' 86th-minute penalty ensured a 1-0 victory against Peterborough United.
Sheffield United are now six points clear of danger after Nigel Clough's men moved into the top half with a 1-0 triumph at MK Dons thanks to a strike from midfielder Steven Scougall midway through the first period.
Elsewhere, Tranmere Rovers and Oldham Athletic drew 2-2 and Swindon Town were held to a 1-1 stalemate by relegation-threatened Crewe Alexandra.
