No. It's not necessary.
It is not necessary to capitalize the word mainland. The sentence is fine with mainland in lower case, while North America is in upper case.
It is on the upper North island on the eastern side.
The upper north eastern part of the united states
The upper portion of a fraction is called the numerator.
The upper portion of your heart contains the atria, both right and left.
England controlled the upper portion and the United States controlled the lower portion. The Upper Portion of Oregon eventually became part of Canada.
Manubrium
The third gym is in Goldenrod City. It is located north of the Ilex Forest. The gym is in the upper right portion of Goldenrod City.
The word capitalize (capitalise) is not a noun it's a verb: capitalize, capitalizes, capitalizing, capitalized. To capitalize is to use upper case letters; to convert into capital or to compute the present value of financial resources. The noun form is capitalization.
In north-eastern Egypt at the Nile River Delta flowing into the Mediterranean Sea
Fundus- Rounded, upper part of the uterus.
The upper part of the thermosphere is called the Ionosphere A+ Magnetosphere