After receiving a bye in the opening round, the 12-year-old hammered Etianna Whitaker 6-1, 6-0 and Alicia Williams 6-1, 6-1 before meeting Millie Devey-Smith in the last four.
She got through 6-7, 6-0 (10-7), before easing past Anna Michelle Chesca in the final with a 6-2, 6-1 victory.
Marshall-Sefia also went in the U14 event where again she had a bye in the opening round before defeating Whitaker once more, this time 6-0, 6-2.
Holly Mellors was beaten 6-4, 6-3 in the quarter-finals, with Megan Jones knocked out 6-0, 6-3 in the last four before the Lutonian went down 2-6, 6-3 (10-5) to Martha Williams.
Jones left frustrated once more as Town fail to take their chances during Lilywhites defeat
Goal-shy Luton still searching for a first win after Preston defeat
Jones assures Lockyer and co they are still 'key' members of Luton's squad
Hatters were aware of McAtee's shoulder injury before agreeing to sign Grimsby striker
Jones will go back to full strength for Town's clash with Preston
She is now in the grade two U12 and U14 singles event at the 91st Bournemouth Open starting on Saturday.