Slob started his political career as a member of the city council of Zwolle from 1993 to 2001. In 2001, Slob became a member of the Tweede Kamer for the Reformed Political Alliance when leader Gert Schutte retired. After the 2002 elections, the newly formed ChristianUnion lost a seat, and Slob did not return to parliament. When lijsttrekker Kars Veling stepped down as a result of the election loss, Slob took his place. He was put on the second place of the list of ChristianUnion candidates for the 2003 elections, and was reelected.
Slob is currently the fractievoorzitter of ChristianUnion. He was also a member of the Parliamentary Committee on Infrastructural Projects