Do I need to obtain a visa prior to visiting Malawi as a US Citizen?

Yes, United States nationals are among those who need a visa when going to Malawi, according to the Embassy of the Republic of Malawi web site

Application Requirements To apply for a Visa, the Passport must be valid for at least 6 months beyond the date of intended departure.

TYPES OF VISA | COST: Tourist and Business Validity

• Transit (7 days) US $70.00
• 3 months single entry from date of issue US $100.00
• 6 months multiple entry from date of issue US $220.00
• One year multiple entry from date of issue US $300.00

Application Requirements

• 2 application forms Visa Application Forms
• 2 Passport-size photos
• Fees can be paid by money order, bank certified check or cash (The Embassy will not accept personal checks as a form of payment)
• Onward ticket
• Valid Passport (at least six months before expiry date with enough unused pages)
• Copy of a letter from the applicant’s Organization or invitation letter.
• Copy of sponsor’s Residential Status in Malawi (i.e. Permit or Malawi Passport if Malawian)
• Return Pre-Paid Envelope addressed to the applicant.

Please note: Application and all required documents MUST be sent in DUPLICATE to the Consular section at the Embassy. In most cases, applications will be processed in 5 working days. However, for nationals of India, Nigeria, Bangladesh, China, Burundi, Rwanda and Pakistan, applications may take up to 4 weeks. Drop off time: 09:30 am to 12:30pm from Mondays to Thursdays.

You don't need to obtain one beforehand, you can still get one on arrival according to Timatic

You do need a visa however, according to the Malawi embassy in Washington and they don't mention anything about visas on arrival. You could always request one before leaving to be safe, as the processing time is just 5 days. They've introduced visa requirements for Europeans and Asians and for those who can get one on arrival it's unclear what the procedure is

The Government of Malawi will issue visas within Malawi but procedures for obtaining them have not yet been published.

Visas on Arrival are available only in cases where a standard visa can not be obtained (ie in countries without a Malawi consular post, extended travel away from your country of residence, etc).

Approval for a VOA needs to be obtained before you arrive from the Immigration Department in Malawi. You can't just simply hop on a plane and be granted a VOA.

As the US has a Malawi Embassy, you likely will be limited to getting the visa through them.

http://www.immigration.gov.mw/visa.html

I am an American that live in Malawi

Visas as issued at the International Airport upon arrival in Lilongwe

Visas can be obtained at the Border crossings by filling a one page Visa document

There are several types of visas but please note that although your Visa may be valid for 3, 6 or 12 months (single or multiple entry) you must check in at the nearest Immigrations Office after 30 days in the country.