Omaze, a for-profit company, partners competitions with charities and celebrity endorsements using a sweepstake system, offering large value homes as prizes.
Founded in 2012 in the US, it says it at least 17% of the money raised through its prize draws goes to charities.
Roper said he had been taking part in prize draws with Omaze for four years before his big win.
Looking ahead, he added: “This win has changed everything for us. I don’t know what we’ll do long-term yet, but we’ll definitely come and enjoy it and host our friends and family.
“I’ve got fantastic options available now, each one something I could never have imagined. I might even retire early.”
Roper, who is originally from Lancashire, started his own milk round at 21 and ran it for 10 years before returning to education to study mechanical engineering.
He has spent the last 14 years working in engineering sales.
“I never dreamed I’d end up owning a house like this – I’ve gone from a milk float to a mansion,” said Roper, whose new property also features £170,000 worth of furnishings, gardens, stables and a barn.
“This time last week I was in the office staring at spreadsheets, now I’m standing by my very own swimming pool and tennis court.
“It’s absolutely crazy, what a difference a week makes.”