TVLine reports that Havins will play the regular role of CIA analyst Esther Katusi in the ten-part series.
The character was previously described as "an optimist" and a Christian with "a strong faith in mankind". In October, Greek star Amber Stevens was linked to the role.
Havins previously starred in ABC soap opera All My Children and has recently featured in episodes of CSI: Miami and Law & Order: Criminal Intent.
Former Dollhouse star Lachman will make a guest appearance in the season premiere as a fellow CIA agent. The actress has made recent guest appearances on Hawaii Five-0 and NCIS: Los Angeles.
The pair will join series regulars John Barrowman and Eve Myles, as well as new cast additions Bill Pullman, Mekhi Phifer and Arlene Tur.
Torchwood: The New World will air on BBC One in the UK and Starz in the US.