He will also serve on the board's leadership, Nomination and Remuneration; and Diversity and Inclusion committees, Cyient said in a statement.
The appointment became effective from April 20, 2017.
"He had a very successful track record in the corporate world and provided distinguished service to the US government. I am confident that he will bring significant new perspectives and guidance to our board and leadership team," Cyient Founder and Chairman BVR Mohan Reddy said.
The development comes at a time when the US, the largest market for Indian IT firms, has proposed a revamp of its work visa regime.
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On his appointment, Thummalapally said he was impressed with Cyient's growth, diversification and global track record in the engineering and technology services industry.
As US Ambassador to Belize from 2009 to 2013, Thummalapally helped to strengthen bilateral ties between the two countries in the areas of economic development, education, citizen security and healthcare.
Thumallapally, thereafter, served as Executive Director, SelectUSA, part of the International Trade Administration under the US Department of Commerce.
SelectUSA was established as a government-wide initiative to attract and retain business investment.