Joan Smith of the media reform campaign group Hacked Off said the sentences had a "symbolic importance" beyond the individual penalties.
"I don't think the length of the sentences really matters very much," she told CNN. "It's the fact that a court has said that this is not just unacceptable, but against the law.
"So it means as a society we are saying that we don't think that this behavior should happen and it will be punished."
Retrial on additional charges
Coulson faces a retrial on two charges of conspiring to commit misconduct in a public office after the jury was unable to reach a decision.
The newspaper's ex-royal editor, Clive Goodman, also faces a retrial on the same charges. Both have pleaded not guilty.
Coulson resigned from the Sunday tabloid in January 2007 after its then-royal editor, Goodman, and Mulcaire were jailed for hacking into voice-mail messages left for royal aides.
Coulson said he knew nothing about the hacking but resigned because he was editor of the paper at the time.
In that July, then-opposition leader Cameron hired Coulson as his director of communications. Cameron became British Prime Minister in 2010, and Coulson moved with him to Downing Street.
In January 2011, Coulson resigned from his post as coverage of the phone hacking scandal broadened. He insisted he was innocent but said he had become a distraction for the government.
Cameron apologized in Parliament last week for hiring Coulson, saying it had been "the wrong decision."
Another of Murdoch's former newspaper chiefs, Rebekah Brooks, was cleared of all charges after the eight-month trial at the Old Bailey court. Her husband and three others were also cleared of all the charges against them.