Speaking at this year's Keswick Convention, she told Christians that politics was no longer a well-respected arena to work in following the recent MPs' expenses scandal.
Baroness Cox, who was deputy speaker of the House of Lords from 1985 to 2005, said the scandal made it "an even better time for young Christians to go into politics to make a difference".
"Parliament has got a bad name – all the more reason to go into it and change it," she said.
Baroness Cox advised, however, against young Christians going straight into politics from university.
"One of the problems with some of our politicians is that they have never done anything else but politics," she said.
"I think it is helpful to have more life experience of some profession or business or other experience.
"There is a terrifying responsibility in politics. You are making laws that are going to influence the lives of millions of people for years to come, and so to have people in politics who will understand the impact of those laws in different areas can be very valuable."
Also speaking the Convention was Dave Fenton, veteran youth worker and Anglican clergyman, who said churches needed to integrate young people if they were to stop them from leaving the church fold before adulthood.
He suggested children and young people could get involved in areas like praying, taking the collection, helping with refreshments and greeting on the doors.
He said: "Young people must be seen and acknowledged, they must take part. We need to have a model that says to young people 'You're involved here'."
