David Buchanan, 18, was ordered to do 200 hours of unpaid community work when he appeared at Guilford Magistrates' Court.
He had admitted two counts of hacking and one of impairing the operation of or hindering access to a computer.
The parenting site had to reset the passwords of 7.7 million members.
Buchanan, of Vann Road, Fernhurst, Surrey, redirected the Mumsnet homepage to a now suspended Twitter profile during his cyber attack last August.
He also admitted hacking into his school's internal website.
Mumsnet was also subjected to a distributed denial of service (DDoS) attack, where an attempt is made to force a site offline by swamping it with internet traffic.
Its users were made to reset their passwords as a "precautionary measure" following the attacks.
The network's founder Justine Roberts, told the BBC Buchanan "caused a lot of angst. It was pretty scary, pretty worrying."
She said the teenager posted members' email addresses and passwords online, which others could access.
Ms Roberts said she hoped he would now "use his talent in a productive, not destructive way".
A teenager who launched a cyber attack against parenting site Mumsnet - forcing it to reset millions of passwords - has been sentenced.