Peter Waters, chair of governors, said Ms Maskatiya has decided to leave headship to become one of Her Majesty’s Inspectors of Schools.
He added: "It is with a combination of pride and sadness to inform you that Ms Yasmin Maskatiya is leaving Chichester High School in December to take on a national role.
"(Her new) role will enable her to influence educational standards nationally. We are immensely grateful for the transformational and inspirational contribution that Ms Maskatiya has made at the High School.
"She led the girls’ school to outstanding and ensured the smooth and successful merger of the two high schools. She is admired and respected across the Kemnal Academies Trust (TKAT)."
Deputy CEO of TKAT, Elizabeth Harrison said the trust owes Ms Maskatiya a 'significant debt of gratitude' for her achievements at the High School, in other TKAT coast schools and across TKAT as a whole.
She added: "Not only has she influenced schools across TKAT but she has also made a significant impact on other schools in West Sussex. All at TKAT wish her every happiness and success for the future."
Chichester High School announced that Joanne McKeown will be the new full time head teacher.
Mr Waters added: "We are sure that staff, pupils and parents will be pleased to know that Mrs Joanne McKeown has, following a selection process, been appointed as full time, substantive permanent headteacher of the school.
"In her current role as head of school, Mrs McKeown, as well as having day-to-day responsibility for leading the school, has had substantial experience acting as headteacher when Ms Maskatiya has fulfilled her executive role across TKAT.
"The governors and TKAT have every confidence that Mrs McKeown will be able to build on the school’s successes."
Mr Waters said 'highly skilled and dedicated leader' Mrs McKeown has worked closely alongside Ms Maskatiya for a number of years and played an 'active role' in the girls’ school journey to outstanding and the 'subsequent successful' merger.
"There will be formal opportunities to thank Ms Maskatiya but I am sure you will join us in congratulating her and wishing her well in her new role," he said.
"In Mrs McKeown, we have an able successor who has been preparing for this role for a number of years. We look forward to the continued success of all those associated with Chichester High School."