The confirmation came this afternoon after a six-month shut down.
The plant was de-activated on August 12 last year, initially for a few weeks to carry out repairs to two of its three boilers.
That timescale was then extended as ExxonMobil decided to carry out a £140m improvement programme.
Hopes of a return to operations before end of the year were dashed, and work to re-start the petro-chemical plant has been on-going for some time.
Jacob McAlister, plant manager, confirmed its return to operations as Mossmorran Action Group (MAG) planned to staged a fourth public meeting tonight.
He said the re-start “was executed safely at each and every stage.”
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He added: “ In re-starting equipment after an extended period, we took the time required to diligently test and re-test processes and safety procedures.
“While I am sure we would all have liked to have returned to production more quickly, we will never compromise the safety of our staff or the public.”
Mr McAlister said the company also took a series of actions “to significantly reduce the rate of the required elevated flaring.”
But he could not rule out further flaring which remains an important part of its operations.
He added: ”We will implement any learnings from every part of the re-start process so that we continually improve, and further reduce elevated flaring in the future.
“As a safety mechanism, our flare remains a vital part of our operations and so we cannot rule out the need to use it in the future. However, the additional work we have conducted during the temporary shut-down is designed to help improve operational reliability and reduce the requirement for unplanned flaring.”
Mr McAlister said the £140 investment programme would “see Fife benefit from a further 850 skilled personnel coming to work at our facility.”