He was 16 when he joined the Operation Mobilization (OM), an evangelical missions movement, and served them for eight years in the Indian subcontinent.

[9] He continued to have close working relationship and friendship with the late George Verwer, OM's founder.

Eventually he also was conferred an honorary degree of divinity by Hindustan Bible Institute and College in Madras, India.

Six months into his undergraduate degree, Yohannan[13] became an ordained clergyman and served in the clergy of a Native American Southern Baptist church for four years near Dallas, Texas.

In 1981, he started a chapter of Gospel for Asia (GFA) in Kerala, India, and in 1983 created an Indian headquarters in Tiruvalla.

The organisation’s primary mission fields include those that live in the “10/40 Window”, referring to the longitudinal coordinates of areas in west Africa, India and east Asia.

[10] Yohannan credited his early work in his native India as inspiration for his focus on the poor and underserved in this region.

[28] In November 2020, the India Income Tax Department raided the residence and offices of KP Yohannan, seizing Rs 57 Lakh (5,700,000 Indian Rupees, about $78,147 US) from a car boot.

[34] It was further alleged that the rubber estate, which Believers Church purchased from Harrison's Malayalam, Ltd., was on government leasehold and therefore not saleable.

At a later time, Harrison's Malayalam was accused of forging their land title, leading to continued debate about the legality of the sale.

[37] However, it also was reported that "The government does not need permission from K P Yohannan to set up airport in the Cheruvally estate, BJP national executive member V Muraleedharan said.

[39] Further, the rubber estate is an investment to help fund social services among underdeveloped communities[33][40] and not a personal land grab as opponents have claimed.

A long standing controversy for Believers Church comes to an end with a clean verdict from the high court of Kerala.

[45] One of these lawsuits went to arbitration and the other was settled after three years in which both parties agreed that “all donations designated for use in the field were ultimately sent to the field.”[46] Gospel for Asia denies any wrongdoing.