This is a headache because of revisions, publishing dates, numbering, and opus numbers which are tangled. Larry Todd (the most eminent Mendelssohn scholar) assigns these dates as being most chronologically relevant:
Symphony Nr. 1, 1824
Symphony Nr. 2, “Lobegesang”, 1840
Symphony Nr. 3, “Scottish”, 1842
Symphony Nr. 4, “Italian” 1833
Symphony Nr. 5, “Reformation”, 1830