Mr Bonneville, who plays the Earl of Grantham in ITV1's popular period drama, lives 40 miles south of London in a village with "pretty pathetic" broadband.
"I yearn for a connection that just stays on, never mind one that goes at the speed of light, he said.
"I'd love to run aspects of my work from home; I'd love to sign up for services that stream content but the connection's just too unreliable.
"Some days I can't even send an email and there doesn't seem to be much I can do about it."
Mr Bonneville is supporting Can't Get Online week – a seven-day roadshow running from 30 October to 6 November.
Organised by the Country Land and Business Association, the week will see IT expert John Popham help rural communities across England get online.
Mr Popham will tour the country, visiting villages and rural communities with poor or non-existent broadband provision.
He will ask residents to sign a 5ft petition calling on the government to increase the benchmark broadband speed to 5Mbps.
Mr Popham will also interview residents about their frustrations getting online and demonstrate how faster broadband can transform people's lives.
The CLA is lobbying to ensure that every rural business and household can access a connection of at least five megabits per second (Mbps).
CLA president William Worsley said: "One fifth of rural England still does not have adequate capacity broadband, and in today's world that is just not acceptable.
"The government wants us to fill in tax returns online because it's more cost-effective and schools expect children to do homework online.
"These are only possible if homes and businesses have a good broadband connection."
Mr Worsley's ancestor Frances Worsley married the real-life Lord Grantham in the 18th century.
"Internet service providers must concentrate on bringing broadband not-spots up to speed rather than making the already fast even faster," Mr Worsley said.
Anyone who would like to sign the petition can attend one of the events or, alternatively, sign online by clicking here.