The television presenter, who hosts 'The Choice' in the United States, met the Irish comedian in 2002.
They reportedly hosted an engagement party for friends and family on Sunday 29.07.12) at London's The Goring Hotel, where Cat, 35, proudly showed off her diamond ring.
A source told the Daily Mirror newspaper: ''Cat and Patrick have been friends for years, so even though they've only been dating a few months, they know they've found The One in each other.
''They wanted all their friends to know they were engaged straight away.''
Patrick, 41, moved into Cat's £1.7million Beverly Hills, California home in May after the couple were spotted together for the first time in February.
In June, Cat admitted she loves how fans were now giving them different dual nicknames.
She said: ''People call us Pat-Cat or Catric. And there is another one which my friends have come up with which is called Caddy, because he is called Paddy.''
She also admitted that she doesn't hang out with other British people In America.
She moved to the US in 2006 to host 'So You Think You Can Dance' and says she went Stateside to have a new ''adventure'' and experience new things.
She said: ''You know what my whole idea when I went out there was to have a bit of an adventure, there's no point in going somewhere just to hang out with Brits, go and have an adventure. But I'm really enjoying it there, I do a job I love then I can go to the beach or go horse riding. I surf, badly.'''