Step 1: List the early Chief Ministers of united Andhra Pradesh in order.
The state's CMs began with Neelam Sanjeeva Reddy in 1956, followed by Damodaram Sanjivayya, Kasu Brahmananda Reddy, P. V. Narasimha Rao and Jalagam Vengala Rao.
Step 2: Count down the sequence to the sixth term.
Going one by one through this order places the sixth CM right after Jalagam Vengala Rao's predecessor in the list.
Step 3: Confirm the name at position six.
That sixth Chief Minister was Marri Chenna Reddy.
\[ \boxed{\text{Marri Chenna Reddy}} \]