The 37-year-old almost quit the sport after leaving London Broncos earlier this year.
But Dons supremo Carl Hall persuaded the former Leeds and Hull FC forward to pen a one-year deal with the Dons.
“I was probably going to retire because I thought I was staying at London but that’s not worked out so I spat my dummy out a little bit,” said Thackray, who was briefly on loan at Doncaster in 2008.
“I thought, you know what, I’m going to retire, I’ve had enough. But I’m excited by the challenge now and I’m looking forward to it.
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“I know Carl from way back as a player. I remember being at Leeds and coming on loan for a couple of games and I enjoyed it then and Carl sold it to me, he’s signed some good players for next year.
“The club didn’t do too badly last year. It’ll be good to play alongside Feka Paleaasina, and there’s a genuine chance of pushing for the top two.”