One way that Mumbi can be seen as symbolic of Africa's struggle for identity is through her changing relationships. As Mumbi's identity is defined by her relations with two men, so is Africa's identity defined by its relationship with two different ways of life.
The first way of life that defines Africa consists of the traditional tribal lifestyles and customs that grew up through countless eons. True, there were internal rivalries and revolts, as when the men overturned the ruling power of women in Thabai, yet these were internal to the traditional order: actions and enmities were known and recognized.
The second way of life that defines Africa is that which was forced upon it by the intrusive "whiteman," whose skin had been so scalded that the "black outside had peeled off" and who brought the message of a god who "let himself be nailed to a tree." Behind the "laughing face" of the white man came a "long line of other red strangers" carrying swords. These were all foreign elements of an unknown order, yet they flourished and grew as the fruit of colonialism.
For Mumbi, her marriage to Gikonyo defined her identity in her beloved traditional order: she was happy with Gikonyo and went where he went. During Gikonyo's imprisonment, Mumbi was defined by the alliance with Karanja that was forced upon her: he brought a new order and identity that flourished around her as the son she had by Karanja grew.
Mumbi's identity is defined a third time by her second relationship with her husband when Gikonyo returns to her after six years of imprisonment. Her traditional order and identity returned with Gikonyo, but changed and distorted. In the same way, independence from the British brought a return of tribal identity and traditional order to Africa, but it was changed, distorted and corrupted.
Yet, as Mumbi and Gikonyo had hope of prospering in their renewed relationship despite the changes, so Africa had hope of prospering in its restored traditional identity and freedom despite the corruption redefining it.
Mumbi is symbolic of Africa in A Grain of Wheat largely because she is an innocent bystander in all of the transgressions.
As Kihika's sister, one would think she would hold a place of high regard within her village and within the Kenyan struggle for independence. However, she becomes a mere pawn and victim in the local power struggle. While her husband, Ginkoyo, is imprisoned in the British concentration camp, she is raped by Karanja, a British collaborator, and the individual rumored to be the traitor who revealed her brother's identity leading directly to his hanging.
Mumbi is left almost without an identity through these struggles, although none of it is her fault or even the result of her actions. Much like Africa itself, she is left ravaged and forced to carve her own identity in a new and confusion world.