Did the Arabs know about Allah before the Prophet (peace and blessings of Allah be upon him) was sent?

You should note that Arabian societies before Islam were not atheistic societies that denied the existence of Allah, or societies that were unaware that there is a Lord, Creator and Provider. They knew that, and they still retained some traces of the religion of Ibraaheem, and they had contact with Jews and Christians. But their problem was that they did not worship Allah Alone in exclusion to others; they had other gods whom they associated in worship with Him, and which they worshipped not on the basis that they were the Lord, the Creator the Provider, but because they claimed that these were intermediaries who would intercede between them and Allah and bring them closer to Allah. Hence Allah said concerning them (interpretation of the meaning):

“And if you (O Muhammad) ask them: ‘Who has created the heavens and the earth,’ they will certainly say: ‘Allah’” [Luqmaan 31:25]

 This indicates that they acknowledged that Allah is the Creator. Another aayah says (interpretation of the meaning):

“And verily, if you ask them: ‘Who created the heavens and the earth?’ Surely, they will say: ‘Allah (has created them)’”

[al-Zumar 39:38]  

Many aayaat indicated that they believed in the Unity of Lordship (Tawheed al-Ruboobiyyah); their shirk was with regard to divinity (uloohiyyah), as Allah says concerning them (interpretation of the meaning):

“And those who take Awliyaa’ (protectors, helpers, lords, gods) besides Him (say): ‘We worship them only that they may bring us near to Allah’” [al-Zumar 39:3] 

i.e., they said, we only worship them so that they may bring us closer to Allah.